Method and apparatus for debris mitigation for an electrical discharge source

ABSTRACT

Method and apparatus for mitigating the transport of debris generated and dispersed from electric discharge sources by thermophoretic and electrostatic deposition. A member is positioned adjacent the front electrode of an electric discharge source and used to establish a temperature difference between it and the front electrode. By flowing a gas between the member and the front electrode a temperature gradient is established that can be used for thermophoretic deposition of particulate debris on either the member or front electrode depending upon the direction of the thermal gradient. Establishing an electric field between the member and front electrode can aid in particle deposition by electrostatic deposition.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT OF GOVERNMENT INTEREST

[0002] This invention was made with Government support under contractno. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to SandiaCorporation. The Government has certain rights in the invention.

FIELD OF THE INVENTION

[0003] The present invention pertains generally to method and apparatusfor mitigating the transport of debris generated and dispersed byelectrical discharge sources used, in particular, for generating extremeultraviolet radiation by the use of thermophoretic and electrostaticdeflection and deposition.

BACKGROUND OF THE INVENTION

[0004] Photolithography is a well-known technique for applying patternsto the surface of a workpiece, such as a circuit pattern to asemiconductor chip or wafer. This technique has the advantage of beingable to faithfully reproduce small and intricate patterns. Traditionalphotolithography involves applying electromagnetic radiation (light) toa mask having openings formed therein (transmission mask) such that thelight that passes through the openings is applied to a region on thesurface of the workpiece that is coated with a radiation-sensitivesubstance, e.g., a photoresist. The mask pattern is reproduced on thesurface of the workpiece by removing either the exposed or unexposedphotoresist. The capabilities of conventional photolithographictechniques have been severely challenged by the need for circuitry ofincreasing density requiring higher resolution features. This isparticularly true for advanced or next generation lithography where thegoal is to produce circuits whose critical dimensions are below 0.1 μm.The demand for smaller feature sizes has inexorably driven thewavelength of radiation needed to produce the desired pattern toever-shorter wavelengths, i.e., toward extreme ultraviolet (EUV) or softx-ray radiation. Two frequently used sources of such radiation are alaser-produced plasma (LPP) and synchrotron radiation. Laser plasmasources are as bright as their more expensive synchrotron counterpartsand are better suited to a small laboratory or commercial environment.The plasma, produced by directing a laser at a target composed of acondensed gas such as krypton or xenon, as it expands through asupersonic nozzle into a vacuum chamber, has been shown to produce astable source of extreme ultraviolet (EUV) radiation, i.e., light whosewavelength in the range 3.5-15 nm. The generation of EUV radiation bymeans of a LPP has been described in U.S. Pat. No. 5,577,092 “ClusterBeam Targets for Laser Plasma Extreme Ultraviolet and Soft X-rays”.Thus, a laser produced plasma (LPP) is well suited for producing the EUVradiation required for next-generation lithography tools and the use ofa LPP for EUV lithography has been described in U.S. Pat. No. 6,031,598“Extreme Ultraviolet Lithography Machine”.

[0005] While generation of EUV radiation by means of a LPP reduces oreliminates many of the problems associated with other sources of EUVradiation, such as production of damaging atomic and particulate debris,there are still significant problems associated with LPP radiationgeneration. Chief among these is the cost, primarily driven by the costof the laser diodes required to pump the lasers. Moreover, the laserpump poses severe technological problems.

[0006] In an attempt to overcome the difficulties associated with LPPradiation sources, Silfvast, in U.S. Pat. No. 5,499,282 “EfficientNarrow Spectral Width Soft X-ray Discharge Source”, describes a pulsedelectrical capillary discharge source. The electrical discharge sourceemploys a pulsed high voltage, high current electric discharge in a lowpressure gas to excite a plasma confined within a capillary bore region.Any gas that can be ionized to generate a plasma that produces radiationat the appropriate wavelength can be used. For generating EUV radiationand soft x-rays xenon is the most desirable species.

[0007]FIG. 1 is a cross-sectional view of a typical electrical dischargesource 100 that comprises generally an insulating disc 110, typicallyfabricated from a ceramic material, having an axial capillary bore 115,a front electrode 120 and a rear electrode 130, disposed on either sideof insulating disc 110, each having an aperture aligned with capillarybore 115. In operation, a gas, i.e., Xe gas, flows through capillarybore 115 from rear electrode 130 to front electrode 120. A high voltage,high current electrical pulse is established across the front and rearelectrodes by power supply 140 causing electrons in the gas to beaccelerated and to collide with and excite gaseous atoms causing them toemit radiation. In the case of Xe gas the radiation is extremeultraviolet (EUV) radiation at about 13.5 nm. However, this dischargesource ejects significant amounts of debris eroded from the capillarybore and electrodes.

[0008] The intense plasma generated in the capillary bore tends to heatthe capillary walls and, depending upon the material used, causes thesurface of the capillary bore either to vaporize or to repeatedly meltand solidify. Furthermore, significant stresses are introduced near thesurface of the capillary bore by the intense thermal gradients generatedduring the discharge cycle. The combination of these stresses andchanges in the physical state of the capillary bore surface causematerial to break away from the surface and generate debris. It is thisdebris that can coat and erode the surface of proximate opticalcomponents used to collect EUV light, thereby severely affecting theirreflectance and reducing their efficiency.

[0009] Debris generation remains one of the most significant impedimentsto the successful development of electrical capillary discharge sourcesfor EUV lithography and various modifications of the basic electricaldischarge source design have been proposed to block debris from reachingcritical optical features. As illustrated in FIG. 2, these prior artmodifications generally employ physical means, such as reconfigurationof front electrode 120. While providing a direct blocking path fordebris travel, debris emitted in an angular distribution from thecapillary can still escape. Moreover, these modifications also intercepta significant portion of the generated EUV radiation, permitting onlycollection of that radiation that is emitted in an angular directionfrom the capillary.

SUMMARY OF THE INVENTION

[0010] Accordingly, the present invention is directed, in part, to amethod and apparatus for mitigating the transport of debris generatedand dispersed by an electrical discharge source. The method is based onthe use of thermophoretic and electrostatic deposition to prevent bothcharged and neutral particles from leaving the capillary dischargesource and depositing onto proximate collection optics degrading theirefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings, like elements with like functions are referredto by like numbers.

[0012]FIG. 1 is a schematic illustration of a generalized prior artelectrical capillary discharge source.

[0013]FIG. 2 shows a modification of a prior art electrical dischargesource.

[0014]FIG. 3 is a schematic embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention is directed to an improved electricalcapillary discharge source, wherein the improvement provides fortrapping of debris generated by the capillary discharge source bythermophoretic and electrostatic deposition.

[0016] Referring now to FIG. 3 which illustrates a schematiccross-sectional view of an embodiment of the electrical capillarydischarge source 100. As in the prior art discharge source illustratedin FIG. 1, the present electrical discharge source comprises aninsulating disc 110, typically fabricated from an electrical insulatormaterial such as a ceramic material, having an axial capillary bore 115,a front electrode 120 and a rear electrode 130, disposed on either sideof insulating disc 110, each having an aperture aligned with capillarybore 115. Xenon gas flows through capillary bore 115 from rear electrode130 to front electrode 120. A high voltage, high current electricalpulse is established across the front and rear electrodes by a source ofelectric potential 140, such as a power supply capable of generatingpulses. Typically, the gas pressure near the rear electrode is about1-20 Torr while at the front electrode it is in the range of about 1-50mTorr. The lower gas pressure in the region of the endcone is desirableto reduce EUV absorption by residual gas.

[0017] An endcone 150, comprising a conical-shaped surface of rotationsubtending an angle of about 20° to 30° from the axial line of capillary115 in order to avoid blocking more than a small solid angle of theradiation emitted from the discharge source, is disposed proximate frontelectrode 120 and on the axial line of capillary 115. The endcone can beheated to a temperature higher than that of the front electrode andcapillary to produce a temperature difference between the endcone andthe proximate front electrode. Xenon gas flowing in the space betweenendcone 150 and front electrode 120 at a pressure of about 1-50 mTorrcan support a temperature gradient, thereby creating a thermophoreticforce that will push debris particles away from the hot region (endcone)and toward the cooler front electrode. Alternatively, the endcone can bemaintained at a temperature below that of front electrode 120, forexample by active cooling, thereby establishing a “reverse” thermalgradient, whereby debris particles are caused to deposit onto endcone150. The space or gap between the endcone and front electrode can be inthe range of about 1-10 mm and is determined by such factors as the EUVoutput and the desired thermal gradient between the front electrode andendcone.

[0018] The endcone can be made of an electrically conducting materialand heated by means of an electric current. Alternatively, the endconecan be made of an electrically and thermally insulating material, andpreferably from a high temperature ceramic insulating material such asalumina, boron nitride, that can serve as a support member for anelectrically conducting material. By affixing to the outer surface ofthe endcone (an electrical insulator) an electrical conductor such as ametal cone or metal mesh material 160, capable of withstanding therequired high temperatures, such as Mo or W, the endcone can be heatedby an electric current from a voltage source 170 to a temperature higherthan that of the front electrode and capillary to produce a temperaturedifference between the endcone and the proximate front electrode.

[0019] In the event it is desired that thermophoretic deposition takeplace on the surface of the endcone, the endcone can be hollow andfilled with a coolant such as liquid nitrogen, thereby establishing a“reverse” thermal gradient. As before, the temperature of the surface ofthe endcone can be regulated by means of an electric current applied tothe endcone.

[0020] Thermophoretic forces operate to cause particles in a gas to bedriven from regions of higher gas temperature to regions of lower gastemperature. Thermophoresis can be a useful tool to overcome particledeposition onto surfaces because it is capable of overwhelming thosemechanisms that lead to particle deposition such as: 1) electrostaticforces, 2) inertia, 3) Brownian motion, and 4) gravity. There are twocritical features associated with thermophoresis generally: 1) atemperature gradient must be developed in the gas resident between twosurfaces to cause a thermophoretic force to be developed, and 2) the gaspressure must be sufficiently high to enable sufficient collisionsbetween gas molecules and particles to develop a thermophoretic force.Thus, while thermophoresis vanishes in a perfect vacuum, pressures aboveabout 100 mTorr are sufficient to establish, in most gases, a welldefined temperature gradient, however, useful thermophoretic protectioncan be established at pressures as low as 1 mTorr, although with alowered effectiveness. A general discussion of thermophoretic forces andtheir use to protect critical surfaces can be found in U.S. Pat. No.6,253,464 “Method for Protection of Lithographic Components fromParticle Contamination”.

[0021] Thermophoresis will act on charged as well as neutral particles.By allowing the endcone to electrically float to either positive ornegative voltages an electric field can be established between endcone150 and front electrode 120. Thus, in addition to removing debrisparticles by thermophoresis, the electric field will enhance removal ofcharged particles from the gas stream exiting the capillary bore of thedischarge source by electrostatic attraction.

[0022] The following embodiment is provided as an illustration of theoperation of the invention. However, this invention may be embodied inmany different forms and should not be construed as limited only to theembodiments set forth herein but as defined by the appended claims.

[0023] In the electrical discharge source illustrated in FIG. 3 aseparation between the endcone and front electrode of about 5 mm wasestablished. During source operation, front electrode 120 is generallyat a temperature of about 1500 ° C. To capture as many debris particlesas possible, a temperature gradient of about 400 ° C./mm was establishedbetween the endcone and the front electrode. Consequently, the endconewas maintained at a temperature of about 1700° C. by passing an electriccurrent through mesh 160. A gas pressure in the range of about 10-100mTorr was maintained between the front electrode and endcone duringsource operation. With a 5 mm gap between the endcone and frontelectrode, Paschen's Law provides that a voltage difference of about 1kV can be supported before electrical breakdown of the intervening Xegas would take place. Thus, very effective electrostatic particledeposition can be achieved by providing voltage differences as small as100 V.

[0024] In summary, a method and apparatus are disclosed that uses boththermophoresis and electrostatic deposition to mitigate particulatedispersal from an electric discharge source.

We claim:
 1. An electrical discharge source comprising: an electricallyinsulating body defining a capillary bore having an inlet and outlet; afirst electrode defining a channel having an inlet connected to a sourceof gas and an outlet in communication with the inlet of the capillarybore; a second electrode defining a channel in communication with theoutlet of the capillary bore; an endcone disposed proximate said secondelectrode and on the axial line of the capillary bore, wherein theendcone is maintained at a temperature different from that of saidsecond electrode; and a source of electric potential for generatingelectrical pulses.
 2. The discharge source of claim 1, wherein the gasis at a pressure of at least 1 mTorr.
 3. The discharge source of claim2, wherein the gas is xenon gas.
 4. The discharge source of claim 1,wherein said endcone comprises a material that is an electrical andthermal insulator.
 5. The discharge source of claim 4, wherein theinsulator includes alumina or boron nitride.
 6. The discharge source ofclaim 4, further including an electrical conductor affixed to the outersurface of said endcone.
 7. The discharge source of claim 6, wherein theelectrical conductor comprises a metal mesh material.
 8. The dischargesource of claim 7, wherein the metal mesh material is W or Mo.
 9. Thedischarge source of claim 1, wherein the spacing between said endconeand said second electrode is between about 1 to 10 mm.
 10. The dischargesource of claim 1, wherein said endcone is configured to provide anelectrical field between said endcone and said electrode.
 11. A methodfor reducing dispersal of particulate debris from an electricaldischarge source, comprising: providing an electrical discharge source,the source comprising: an electrically insulating body defining acapillary bore having an inlet and outlet; a first electrode defining achannel having an inlet connected to a source of gas and an outlet incommunication with the inlet of the capillary bore; a second electrodedefining a channel in communication with the outlet of the capillarybore; an endcone disposed proximate said second electrode and on theaxial line of the capillary bore, wherein the endcone is maintained at atemperature different from that of said second electrode; and a sourceof electric potential for generating electrical pulses; establishing atemperature difference between said endcone and said second electrode;and flowing a gas from the source of gas between said second electrodeand said endcone to establish a thermal gradient therebetween.
 12. Themethod of claim 11, wherein the pressure of the gas between said secondelectrode and said endcone is greater than about 1 mTorr.
 13. The methodof claim 12, wherein the pressure of the gas between said secondelectrode and said endcone is in the range of about 1-50 mTorr.
 14. Themethod of claim 11, wherein the gas is xenon gas.